In various mobile radio communication standards, for example the Global Systems for Mobile Communications (GSM) Standard of the European Telecommunications Standards Institute (ETSI), mobile communication terminals are provided for emitting information bursts in specific radio channels with predetermined power levels. These stipulations should be complied with exactly. On the one hand, a transmitter should be switched on and switched off rapidly enough so as not to lose any time that could be utilized for some other purpose, and on the other hand, however, inadvertently transmitting radio signals on radio channels lying adjacent to the desired radio channel should be avoided.
These requirements have the effect that a transmitter should have the following properties:
a high accuracy;
fast switching (fast switching on and fast switching off and also fast changeover);
a good switching profile (in order to avoid an undesired selective widening of the spectrum).
Furthermore, it should be noted that an output stage of a radio power amplifier for example in the case of GSM is sensitive with regard to a mismatch of the antenna(s). In the case of a mismatch with the antenna(s), a current of the output stage of a radio power amplifier can rise considerably depending on the phase. Such a large current brings about a large power consumption, which is of considerable importance particularly in the case of mobile devices. Under extreme conditions, the current can even be so large that the radio power amplifier is irreparably damaged. This also leads to stringent requirements made of the accuracy of the transmitter.
The transmitter is usually controlled by means of a closed control loop.
Analogue control can be used in a simple manner in the case of constant envelope modulation of the output signal, such as, for example, GMSK (Gaussian minimum shift keying, frequency shift keying with a modulation index 0.5 with a Gaussian filter connected upstream). However, it is not suitable for a modulation with a changing envelope of the output signal, such as, for example, in the case of an 8PSK modulation (8 phase shift keying).
For this reason it is desirable to use improved solutions, such as a digital control architecture, for example, which can be deactivated without glitches after an 8PSK modulation ramp-up. In some solutions, the control loop is arranged in a radio transmitter and generally controls the radio power amplifier by controlling the input power in the radio power amplifier (method for 8PSK signals), or by controlling a control voltage, usually referred to as VRAMP (a method generally used for GMSK signals). Since the properties of the radio power amplifiers in mass production vary considerably from item to item, it may be desirable to set specific parameters in order to achieve a good performance even in the case of the radio power amplifiers in the marginal region of the radio power amplifier distribution in mass production.